DIY - Heater Bypass Valve

While I'm waiting for my new triple core Mark Preisendorf radiator to arrive, I thought I'd deal with some other cooling system odds and ends.

Most of us E9 owners are aware that our cars do not have a conventional heater contol valve that shuts off the flow of coolant through the heater core. Instead, the core is always plumbed, and the heater lever merely opens and closes the flaps on the heater housing. In hot weather, many of us can feel the heat coming off the heater housing. The a/c systems in these cars, never great to begin with, can certainly do without this extra heat load right next to the a/c evaporator.

There have been threads ( discussing what is necessary to bypass the heater core. The quick and dirty way is to simply disconnect the two heater hoses from the heater core pipes that protrude through the firewall and splice them together. For those living in hot climates, this is fine, but here in New England, on those spring drives, I USE my heat. A second approach is to install a valve, like in a 2002, that simply stops the flow of antifreeze into the core. However, there is concern that this approach, which does not allowing the coolant to circulate, may cause overheating problems. Clearly what is needed is a full heater bypass valve that, when opened, sends coolant through the core and, when closed, bypasses the core without blocking the flow.

For reference, the hoses into the firewall are shown below (hose clamps removed). The top hose goes to the back of the head. The bottom hose (on my L-Jet M30B32) feeds the metal pipe that runs under the intake manifold.


It is easy to imagine the valve that we need. It is shaped like a letter H. The top legs of the H are connected to the heater pipes at the firewall, and the hoses go into the bottom legs of the H. When the valve is open, it should block off the center section but allow coolant through the legs to and from the heater core, and when the valve is closed, it should bypass the core by diverting the coolant coming up the left leg, through the center section, and out the right leg. But how do you locate one the right size, and what adaptations to you need to do to make it work in the E9?

In a perfect world, we'd find a bypass valve that:
--Has 3/4" pipes (this is the size of the heater pipes protruding through the firewall)
--Has a 1.5" center-to-center (2" edge-to-edge) spacing between the pipes (this is the spacing of the heater pipes protruding through the firewall)
--Is metal

In practice... one out of three ain't bad, and close is good enough for horseshoes and hand grenades.

To locate a usable valve, I went on eBay, typed in "heater control valve," and spent an hour looking at lots of pictures, eventually finding three that are widely available through both the AC Delco and the Four Seasons cataloges. They're all cheap (less than $35). Because it's impossible to tell what will work and what won't until you actualy test-fit them, I simply bought all three. I think the total bill was about $60.


Note that all three of these have a lever on the end of the valve, which is, in turn, actuated via a vacuum dashpot. So in order to change from the heat to the no-heat setting, you could either remove the vacuum dashpot and simply move the lever (or wire it in position), or leave the dashpot installed and connect and disconnect it from the intake manifold to flip between enabled and bypassed.

Note also that, to connect any of these three candidate bypass valves to the two heater hoses, the heater hoses need to be either trimmed or lengthened.

AC Delco 15-5533
Verdict: This should work.
Advantages: The pipes are almost exactly the same spacing as the heater pipes, allowing clean attachment to the pipes on the firewall.
Disadvantages: The pipes are 5/8", not 3/4". And it's plastic.


The first one is AC Delco 15-5533 (Four Seasons 74781). This is not a classic "H" shape, but it has two inlets and two outlets. The one I bought came without the vacuum actuator, so the lever controlling the valve is plainly visible and completely exposed. In this picture, the flow of coolant in is supposed to be from the top right. The tube size is only 5/8" instead of the correct 3/4", but you can snug the hoses down with hose clamps. And, the spacing of the two tubes on the left is about 1.5" center-to-center -- almost exactly the spacing of the two heater pipes on the firewall. Unfortunately, the part is plastic -- an anathema to those of us who pride that our E9s don't have the plastic junk endemic in the cooling systems of newer BMWs.

The pic below is a test fit of this valve using two short 3/4" rubber hose sections to hold it onto the firewall. In order to have clearance for the lever, the lever must go on the left (away from the intake manifold). This forces you to install the valve upside down from the previous photograph, so the inlet side is lower left, where you can just barely see it. The outlet side is plainly visible upper left. The hose going to the metal pipe under the intake manifold is swung to the side. The hose coming from the back of the head is not visible.


In the photo below, I've swung the outlet hose back toward the valve so you can see that the natural elbow in this hose will allow the excess to simply be cut and the hose connected (I have not yet cut it; this is a test fitting). The hose from the back of the head will need to be lengthened with a 3/4" barb coupling and brought into the inlet at lower left (not visible).


So you can see that this should work. If you look up this part number on line, you'll see the vacuum actuator bolts on top of the lever, so it would be on the left. It looks like there's sufficient room for it.

AC Delco 15-5543
Verdict: This should work.
Advantages: The pipes are 3/4". And it comes with hoses on it, allowing quick attachment to the heater pipes on the firewall.
Disadvantages: The pipes are slightly too far apart. And it's plastic.


The next one I tried is the AC Delco 15-5543 (Four Seasons 47607). This looks to be exactly what you need, although, like the 5533, it is plastic. The pipes are 3/4" -- the right size. It's H-shaped with the actuator on the top. It even comes with two 4" sections of 3/4" hose on the ends, allowing quick attachment to the heater pipes. The inlet end isn't labeled, but I assume it's the one opposite the valve, or upper left, In this first pic below, I do a quick test fit. The 4" rubber hoses are longer than they need to be, so it sticks out a bit far.


Below I've trimmed the 4" hoses down to about 2". Unfortunately the pipes on this valve are slightly further apart than those on the firewall, and the shorter you cut these hoses, the more difficult a time they have mating up securely to both sides. Also in this pic, I swing the hose from the intake, showing that, like with the 5533, it looks like this hose can simply be cut just after it makes its elbow turn and then clamped directly to the pipe. Also, as with the 5533, the inlet hose from the back of the head (not pictured) need to be lengthened in order to be connected to the valve. The vacuum actuator can be taken off and the lever flipped manually, or if you like, the actuator can be used to close the valve; just hook a vacuum line to the intake manifold.


AC Delco 15-5302
Verdict: I have my doubts.
Advantages: It's metal.
Disadvantages: The pipes are slightly too far apart. One pipe is 3/4", the other is 5/8". There may be clearance issues.


I looked long and hard for an H-shaped metal valve, and this was the only one I could find. Initially I thought it was The Grail, but the vacuum actuator is on the side, and valve, pipes, and the vacuum actuator sit at an angle with respect to each other. Further, what is marked as the inlet tube is 5/8" whereas the outlet tube is 3/4". Also, the outlet and inlet tubes extend unequal distances. Individually none of these are showstoppers, but they make a clean installation challenging. In the pic below I have the valve cajoled into hanging there on the firewall. You can see that the tubes are slightly too far apart to mate cleanly with the firewall tubes, though slightly longer stub hoses would solve this problem. In this position, this could work, but technically the flow arrow is pointing the wrong way.


In the pic below, we turn the valve around so the flow arrow is pointing toward the heater, and you can see that there's a clearance problem, with the body of the valve hitting the #6 intake plenum.


So, it looks like either of the two plastic bypass valves -- the 5533 and the 5543 -- will work. As much as I dislike putting plastic in the cooling system, unless I can find a better metal bypass valve, I'll likely go with one of these. Plus, the color and the look and feel of the metal valve, to me, are somewhat at odds with the rest of the rest of the engine compartment. Of the three, interestingly, the 5533 seems to be the least visually intrusive. The 5543 is a quicker and easier adaptation, but that vacuum dashpot is sitting pretty high and looks fairly un-BMW-like (of course you could take it off). On the 5533, the dashpot unscrews, but on the 5543, it is a more integral part of the assembly, not as easily removed.

To finish up the adaptation and extend the inlet hose from the back of the head, I've ordered a length of Gates 5/8" heater hose (28491, $10.03 at Amazon) and Gates 3/4" heater hose (28492, $9.64 at Amazon), and several brass right angle 5/8" and 3/4" fittings (eBay; PEX fittings look like they'll work).

I'll amend the post when the final installation is done.

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Why not the E34 method?

If someone else does it this way, I'd love to see the result. I gave it a quick look-see and opted out for several reasons. Firstly, there were posts on this and other forums that said that simply blocking the flow (like is done with the heater control valve on a 2002) would be bad for the cooling of the rear cylinders of the engine, and because the heater control setup of an E34 has three hose fittings not four, I couldn't convince myself that it bypasses in the way that I wanted (of course you could also argue that, however it -- or the bypass setup of an E24 -- worked, it meant that that technique was unlikely to overheat the engine or they wouldn't have used it).

Also, that E34 valve ain't small. I wanted something that didn't eat up a lot of space and didn't dominate the look of the engine compartment.

Lastly, I wanted to keep it as simple as possible. This is a lever that'll probably be flipped once a season.
Heater Control Valve Installation: Part II
For some reason, on my computer the pictures are not rendering in this article (Heater Control Valve Installation: Part II). Does anyone know why or does someone have a version with pictures? Other pictures in other threads are displaying fine. Or is there another thread that's more recent that takes another approach?

HB Chris

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For some reason, on my computer the pictures are not rendering in this article (Heater Control Valve Installation: Part II). Does anyone know why or does someone have a version with pictures? Other pictures in other threads are displaying fine. Or is there another thread that's more recent that takes another approach?
Rob linked his and did not paste them in. Other pics on page 4 show up.


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Heater Control Valve Installation: Part II
I would like to try this method of bypassing the heater core but the pics in your post have vaporized. Can you repost them or suggest where to find them?
Thank you,


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(This is the repaired version of my post from Jan 16, 2014, with the broken links replaced with directly embedded photos. It's broken into two posts because, for some reason, it's now flagging it as being too long. Never let it be said that I don't love this forum and this car :^)

Adventures in Grommet Land -- Electric Bypass Valve Installed (part I)

Okay, I have the electronic heater bypass valve installed. This is the Old Air Products part number 50-0155 ( It’s an electronically-controlled servo-actuated valve. Old Air Product’s price is $86.50 plus shipping, but I found one, NIB, on eBay for $75 so I thought I’d try it.

As per prior posts, I had some trepidation about doing this:
--It’s plastic, and the thought of introducing plastic into the cooling system of an older BMW that had not yet been so infected is an anathema.
--Part of the appeal of old cars with mechanical heater controls is that there are no little electric servos to fail, and this is introducing a little electric servo.

However, the pros are:
--I’d already installed the exact same plastic valve in the car – the AC Delco 15533 (Four Seasons 74781) – but without the servo, so… in for a penny, in for a pound.
--Both the cable-actuated and electronically-actuated versions allow you to crack the valve open just a little, for just a little heat, whereas the vacuum-actuated version is all or nothing.
--The electric servo control seemed less invasive than the manually cable-actuated version of the same valve.
--If the servo failed, it could merely be unbolted and the valve could be operated manually, like the one I already had in there.
--I’d been looking at this for a while, identified it as a solution, and needed to pull the trigger, try it out, and be done with it.

There are actually three assemblies – the valve and motor, a cable that plugs into the motor and goes through the firewall, and another cable that plugs into that one, has a small sealed electronics box, and terminates in the close/open control rheostat.

I first tested the valve to make sure it worked like I thought. This is trivial – wire the red (+) and black (-) wires to 12V and ground and turn the rheostat knob. When you do, you can see the end of the motor turn, and verify it by blowing into the inlet and feeling where the breath comes out. So it works.

Next was verifying that the valve and servo will actually fit. There’s not gobs of room, but it does fit. The connector on the cable comes close to the mount holding the brake booster but does not actually hit it; you just need to plug the cable in before you screw down the hose clamps; it’s awfully difficult to plug in the cable afterward. Also, a more flexible drain sock needs to be fitted. The stock sock will hit the valve, and if the sock isn’t there, rainwater will run directly onto the valve, which probably isn’t the best thing.



By far the most difficult part of the installation was getting the cable through the firewall. As you can see from the pics, the cable has a flat Molex-style connector on the end about an inch wide.


Obviously I was not going to drill a 1” hole, or any hole for that matter, through the firewall on my precious E9. That was an iron-clad requirement. But there’s a trick to these Molex connectors – you can reach in with very fine tweezers, or even a pin, depress the little barbs that hold the pins into the connectors, then slide the pins out the back of the connector. I did this and pulled the connector off. Now I no longer had a 1” Molex connector to pass through the firewall – I had a cable assembly about the diameter of a pencil. Much easier.

But it still has to go through the firewall somewhere. Initially I thought I’d feed it through on the right side, since the car is air-conditioned and the grommets for the a/c hoses had enough room to allow the cable through. However, since both the valve and the control knob need to be mounted on the left side of the car, zigging right only to zig back left seemed like an unnecessary complication, even though there was almost certainly sufficient cable length to do so. So I elected to stay on the left side.

Now, there are three existing grommeted holes on the left side of the firewall – the main wiring harness, the speedometer cable, and the small cable that goes through at the brake master. On my car, the grommet around the main wiring hardness is old and brittle, and the thought of trying to bend or deform or cajole it with a screwdriver or plastic rod to pass a pencil-sized cable through it gave me the heebie jeebies. The grommet near the brake master is so small I wasn’t certain there was enough room. That left the hole and grommet at the speedometer cable (grommet already removed in photo below).


On my car, over the past 27 years I already had passed multiple wires through this grommet to wire in the double relay for the L-Jetronic retrofit, and additional wires for the air conditioning. In the process of doing this, I’d munged up the grommet, poking holes in it rather than threading the wires cleanly through the end of the grommet’s hood. Ah, the impatience of youth. It was unlikely that the existing grommet was sealing well. I thought I’d use this as an opportunity to replace it.

Oh my god, what a pain in the ass.

Here’s why. The portion of the speedometer cable that runs inside the car is thin and flexible, but the portion that runs from the transmission to the base of the firewall has a thick finger-sized rubber coating around it. It’s very difficult to pull a grommet over this coating. It’s possible that when the cables were assembled new, the grommet was put on from the top (speedometer) end, and then the speedometer end was crimped on. Just a guess.

Getting the old grommet off is easy – just cut it. But to install a new one, you first need to find a new one. Problem one – they don’t appear to be available separately from the speedo cable. At least I couldn’t find a part number. Problem two – if you want to thread additional wires, or a whole additional cable assembly, through the grommet, you don’t really want the original grommet. It’s too tight on the outside end. You want something with a larger opening on the firewall side.

The first step in finding a replacement grommet is sizing it. It was difficult to accurately size the existing hole with the cable going through it, so I pulled out the speedo cable. This isn’t rocket science – you unbolt it from the transmission and pull it through – but you do need to take photographs and careful notes on which brake and fuel lines it goes in front of and behind.

With the speedo cable out, I could measure the hole. It seems to be 0.875”, or 7/8”. I went to the local hardware store and bought a standard rubber grommet with a 7/8” outer diameter. It fit the firewall hole very well. But this isn’t really the grommet you want. It’s one thing if you pass a hose of the same size as the inner diameter through it, but if you just pass cables and wires through it, it’ll leak. You want what I would call a “hooded grommet” or a “bellows grommet” with a portion that extends outside the firewall, acting like a hood to seal the wires.

Unfortunately, when you search for “hooded grommet” or “bellows grommet,” you find virtually nothing. Daystar makes the closest, a “universal firewall boot,” but it required a 1.5” hole. Hooded, bellows, or domed grommets are certainly used by many manufacturers to seal cable assemblies that go through firewalls, but not surprisingly I could not find a reference that listed them by size – you’d need to know a priori that, just to make something up, Mitsubishi part number XYZ012345 was such a hooded grommet and had an outer diameter of 7/8”.

By blind luck, I found that I had a leftover bellows from some long-departed clutch slave, whose outer diameter was almost exactly the inner diameter of the hardware store 7/8” grommet I’d test-fitted. Together, they comprised a hooded grommet that would seal the speedometer cable as well as allow a new cable assembly and other stray wires through. Pulling the bellows over the thick part of the speedometer cable was difficult, made easier with a liberal application of soap.

I then passed the five pins and the five wires from the heater valve through the bellows, and then through the grommet. I did the same with the rest of the loose wires from the L-Jet. Once all the wires were through, I applied a light coating of RTV to the outside of the end of the bellows, and slid it into the inside diameter of the grommet in the firewall. A perfect fit. Remarkable.




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Adventures in Grommet Land -- Electric Bypass Valve Installed (part II)

Once I’d safely exited grommetland, I worked inside the car and put the Molex connector back over the pins.

I then need to figure out where and how to mount the rotary knob. On my car, the kick panel below the steering column has three 1 ¼” holes in it. The right-most one is for the defroster switch, but there’s nothing in the other two; I can’t remember what was originally there or even if that panel was original to the car. These holes were way larger than the hole required for the rotary knob. Damn, need some sort of a snap-in adapter. Then I remembered that, way back when, at least one of these holes had exactly that, with a green dummy button in the middle. I looked in a drawer of rarely-used parts, and, incredibly, put my hand right on it. Using a large washer, I adapted the knob so it mounts securely in this snap-in adapter. It’s a tight fit, but it does fit.





With that, the only things left were to attach the small electronic box to the left side of the steering column housing with double-sided tape, wire the red wire into the fuse box, and the black wire to ground.




This is a quick test-fit of the under-dash panel. It's not installed permanently -- I have other work to do down there.


The car won’t be out of the garage for a few more months (New England winter; you understand), but the electric heater bypass valve is completely installed, and neither the firewall nor the under-dash panel were cut in any way.

I'm pleased.
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Thank you for taking the time to find this and repost. The pics are a tremendous help in figuring out how to do this.


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And if you find you don’t want to go to the solenoid route you can just flip it manually for heat whenever you need it, which for me is once or twice a year.


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I'm all for the heater bypass solenoid. Not being one to do it myself when it comes to the combination of plumbing & electrical, I let the pros figure it out. Had VSR install one in Athena in the Winter of 2015/2016. The stipulation was to have it controlled by a yellow button 'fog light' switch. So with this set up the heat is all off or all on. When on the heat level is adjusted by the stock air controls.